Conventionally a thermosetting resin has been mainly used as a matrix resin of a fiber-reinforced composite material. However, in recent years, a fiber-reinforced composite material in which a thermoplastic resin is a matrix has been focused from the standpoints of cost, quickness and easiness of molding, recycle possibility after use, and the like. As a fiber-reinforced composite material in which a thermoplastic resin is a matrix resin, there is one utilizing a random mat. As the random mat, for example, there is a chopped strand mat in which chopped strands obtained by cutting reinforcing fiber strands that obtained by gathering and bundling from several thousands to several hundred-thousands of filaments (more exactly, single filaments) are randomly sprayed. In the chopped strand mat, reinforcing fibers are difficult to be impregnated with a thermoplastic resin, and a mechanical strength developing property when formed into a shaped product of fiber reinforced composite material (hereinafter simply referred to as a “shaped product”) is low.
Patent Document 1 proposes a technology of improving impregnation ability of a thermoplastic resin into reinforcing fibers by slanting a cross-sectional shape of a chopped strand, thereby improving mechanical properties. However, mechanical strength of a shaped product obtained in this technology is not a satisfactory level.
Patent Document 2 proposes a technology of improving impregnation ability of a thermoplastic resin into reinforcing fibers by making the reinforcing fibers into a single filament form, and thereby enhancing a mechanical strength developing property.
Patent Document 3 relates to a fiber-reinforced thermoplastic resin molding material in which a non-woven material produced by wet paper-making is used as a reinforcing fiber. Conventionally, when preparing a non-woven material by a paper-making method, in order to increase dispersion property, reinforcing fibers have been opened up to a single filament form, and used as a non-woven material. However, the non-woven material obtained by a wet paper-making is very bulky, and when the non-woven material is formed into a sheet-shaped molding material, its thickness is compressed into about 1/10. As a result, there has been a problem that when molding the sheet-shaped molding material, the reinforcing fibers tend to return to an original bulky state and a sheet expands (springback). In Patent Document 3, to solve the problem of springback, a non-woven material of bundle-shaped reinforcing fibers including a plurality of single filaments and a sheet-shaped molding material containing the same are formed by, for example, performing wet paper-making using reinforcing fibers that gathered and bundled by a hydrophobic sizing agent. It is described that the non-woven material may contain reinforcing fibers in a single filament form, in addition to bundled reinforcing fibers including a plurality of single filaments.